It has been known that Saiyans hardly, if ever, get sick from things like that of colds, flus, amongst other illnesses. This is due to three things, first being the way they eat, second is their high energy, and third is their cultural society. Each one of these play a very important factor to how strong a Saiyan's immune system is. This will be a look into each of these factors to understand just how such an immune system was obtained by these impressive warriors.
A Saiyan eats very vitamin and mineral rich diet, allowing for such an immune system build up. With the consumption of meat, they also take in the proteins needed to break down the foods and from there it is quickly spread throughout their body's immune system to help fend off diseases common to the human race. With this style of eating and the vastness of their intake, a Saiyan is protected accordingly against that of cardio and pulmonary diseases. Diabetes is not really an issue with them, nor is that of many other diseases such as coughs, colds, flues, heart disease, and so much more.
With their ki being so high, their food intake and rest allow for this to be maintained without undue stress to the body. This high energy, burns off things like that of fat and other harmful airborne diseases in the air due to their powerful auras. These auras act like that of an invisible barrier that bounces back the more harmful rays of the sun and pathogens that are easily spread through the air around a mass of people or those within a forest that is waiting for someone to become the carrier.
Due to living with many races, the Saiyan immune system is stronger against that of other forms of carriers of disease. To handle the vast array of illnesses that could spread through a society, the Saiyans have developed a natural immune system that automatically fights off the more harmful diseases that can easily be spread, like that of AIDS, Bubonic Plague, Influenza, and so much more. These diseases do not bother the Saiyan so much due to their ability to single out off energies or keep in complete knowledge of their own bodies.
Upon traveling to other societies and other planets, their immune systems would automatically gather the needed antibodies to fight off what illnesses may occur. Though they were not completely invulnerable to such things, they had been known to become ill from time to time. Direct poisonings, serious parasites if they ever managed to enter their bodies, and other harmful acts still had to take precautions of it and the Saiyans took such threats seriously. However as for their immune systems, for the most part it was very well built. As stated in Dragonball, it takes a Saiyan two days to overcome an illness once it has taken hold. This holds true for much of the time save for when Kakarrot (Goku) came down with an unknown heart virus that was specifically designed to attack the Saiyan, and also later known to not be contagious.
The Heart Virus is a disease that attacks the heart. Though the name of the antidote is never given, it was inclined that it killed Kakarrot in the "History of Trunks" movie and later spoken before the Android Saga. During the fight with Androids 19 and 20, Goku was seen quickly trying to end a fight however a debilitating agony would soon bring him to his knees. If it were contagious, then Vegeta, having been on the scene, would have contracted it as well. However, he did not concluding such a hypothetical view as fact. In his past, it took 20 years to develop the antidote.
The symptoms have been as such. Nightmares, excruciating pain, high fever, rapid and ragged breathing, severe exhaustion, and an almost comatose state. These symptoms work to wear down the Saiyan's immune system before it takes hold and forces the warrior to become bedridden, making them feel as if they were dying. Doing this is no easy feat, but since a virus is continually mutating, it is easier for such a disease to take hold. Also the genetic makeup would be that similar to the Saiyan's own system, fooling it long enough to mutate and finally show itself in ruthless illness.
How severe are these symptoms to a Saiyan? They can be lethal if left untreated and in ten days can even kill the Saiyan. Nightmares are often of a worst case scenario, or a severe time in their lives where they are under the most stress or pain. A fever for a Saiyan under these conditions would bring the fever to approximately 108 degrees where the effects would require a cold compress constantly. Then it attacks the heart with debilitating ruthlessness. When it attacks the heart, this anguish can be like someone putting their hand right through the chest and grip the heart with relentless agony. This agony is the part that actually weakens the Saiyan often causing an erratic heartbeat.
Next in the series of symptoms is the severe exhaustion and the rapid and ragged breathing. This is the effect of complete and utter exhaustion from the viral attack. With everything put into fighting the pain, the fever, and the virus all at the same time, it will put them bedridden at the very least. This is also the most dangerous time and the time when the antidote is needed in a most crucial time frame. Lack of it will send the message from the brain to the other organs to begin shutting down in an attempt to purge the body of the illness. Once this takes place, the high fever begins to ravage the body even more in the comatose sleep. Should it be left untreated, the vitals start shutting down.
In this final stage the last three to stop are in this order. Lungs which stop the breathing and thus getting precious oxygen to the heart and brain needed for life. Then comes the heart to stop breathing. Life support may be placed on to monitor this stage but once the heart stops, it is known as clinical death. CPR or known as Cardio Pulmonary Resuscitation may be performed at this time for what is known as the Golden Hour. Once the hour passes and if nothing is done, finally the brain dies, causing what is known as biological death. At this point death is not reversible.